FIG. 4 shows such a table-turning driving apparatus. This table-turning driving apparatus is so constituted that a plurality of helical gears of various sizes are provided on shafts 1, 2, 3 and 4 coaxially therewith, respectively, and such gear train transmits the power of a drive motor 7 to the table. Helical gears 9a, 9b and pinions 10a, 10b are mounted on shafts 5 and 6 coaxially therewith, and the pinions 10a, 10b are adapted to engage a ring gear 11 integral with the table not shown.
Such the table-turning driving apparatus requires an adjustment in an amount of backlash and an amount of preload of the gear train in a driving system for smooth operation.
FIG. 5 shows a mechanism for adjusting an amount of backlash in the table-turning driving apparatus shown in FIG. 4. A spline shaft is used as the shaft 2, and two helical gears 12a, 12b having the same dimensions are mounted on the spline shaft 2 coaxially therewith so as to engage spline grooves 13. An adjusting collar 14 is interposed between the gears 12a and 12b to adjust the distance in the axial direction, so that a change in the dimension of thickness of the adjusting collar 14 allows an adjustment in the backlash between the helical gears engaging each other, resulting in an adjustment in the amount of backlash between the pinion 10 on the final shaft and the ring gear 11.
However, such an adjustment in backlash results in an intricate operation comprising many steps of measuring the distance between the helical gears 12a and 12b in a state of temporary assembly, machining the thickness of the adjusting collar 14 according to the result of the measurement and, thereafter, incorporating the adjusting collar 14 into the device to perform a true assembly. This requires a high degree of skill to adjust the amount of backlash to a proper value.
Further, the conventional device has a disadvantage in that the backlash is adjustable only in the direction of reducing the thickness of the adjusting collar 14, and is not adjustable in the reverse direction.
Moreover, since the conventional device has a number of shafts of the gear train and a number of helical gears, it becomes a complicated mechanism having many parts and, therefore, problems in the working properties of assembly and disassembly, maintenance and the like are pointed out.